The invention relates to a process for chemical reduction of the phosphate content of water by adding at least one reagent which forms a crystalline difficultly soluble salt, and contacting the liquid with a seed material promoting the crystallization.
Processes of this kind are known. They are particularly used in the purification of waste water.
Both the municipal and the industrial waste water are nowadays purified biologically on a large scale, and the phosphate is thereby removed only to a limited extent. However, the disposal of this purified waste water still containing appreciable amounts of phosphate may give rise to a promotion of the growth of algae in the water stream in which it is disposed. Therefore it is often desirable to reduce the phosphate content of the biologically purified waste water still further, before it is drained.
The present state of the waste water purification art encompasses three chemical processes for the reduction of the phosphate content, which are classified according to the location in the purification process, where the reagents are dosed, to wit:
pre-precipitation, whereby the reagents are introduced and the precipitate obtained is removed before the start of the biological purification,
simultaneous precipitation, whereby the reagents are introduced during the biological purification and the precipitate is removed together with the biological sludge, and
post-precipitation, whereby the reagents are added and the precipitate obtained is removed after completion of the biological purification.
In practice iron and aluminum salts are frequently used as reagents for the phosphate removal. These salts have the disadvantage that large amounts of extraneous anions (sulphate or chloride) are introduced into the water, thus increasing the salt content thereof. Moreover, these metals remain in the obtained sludge, which reduces the agricultural value of the sludge to practically zero.
These disadvantages do not adhere to the use of lime as a reagent for the removal of phosphate. Until recently, however, the lime precipitation was almost exclusively executed as a pre-precipitation. Then a high pH must be used, so that a simultaneous conditioning of the water takes place, which accordingly produces large amounts of additional sludge. Also it is not allowed to remove all phosphate from the water, since some of the phosphate content is used in the biological purification for the cultivation of the biological cell material. Therefore the reaction with the lime should be controlled carefully, which has been shown in practice to be difficult.
In the simultaneous precipitation method lime can be used, too. Thereby the starting point was an investigation of Jenkins. Menar and Ferguson, described in Applications of New Concepts of Physical-Chemical Wastewater Treatment 1972, pages 211-230. From this investigation it follows that at a relatively low pH an appreciable amount of phosphate may precipitate as hydroxyapatite (Ca.sub.5 (PO.sub.4).sub.3 OH). Prerequisite for obtaining chemical equilibrium in a reasonably short time is that there is present in the reactor a sufficient amount of already formed apatite crystals.
In practice, however, it appeared that although by dosing the lime in the biological step of the purification a substantial increase in the phosphate removal could be secured, the residual amount of phosphate in the water was still higher than desirable and that appreciable amounts of calcium carbonate were formed owing to conditioning of the water.
All processes of phosphate removal used up till now, in which a lime-phosphate-precipitate was formed, have in common that always a sludge is formed of the difficultly soluble phosphates. To be true, in the simultaneous precipitation with lime mention is made of formed apatite crystals, but the dimensions of these crystals are so minute, that they can be hardly if ever distinguished from the biological sludge. This means that the apparatus which is known to this end is bulky and expensive, since the obtained precipitate can only be removed by sedimentation and/or filtration, while moreover large amounts of sludge are produced having a high water content of 95 percent or more. Especially the formation of large amounts of sludge is nowadays seen everywhere as a serious problem, since the disposal of the sludge becomes increasingly more difficult.